Dense molded bodies of polycrystalline aluminum nitride and process for preparation without use of sintering aids

ABSTRACT

The invention is a molded body of polycrystalline aluminum nitride having a density of at least 99.8% TD and comprising: 
     at least 99% by weight AlN 
     up to 0.35% by weight residual oxygen 
     up to 0.35% by weight residual carbon and 
     up to 0.03% by weight metallic impurities (Fe, Si, Ca, Mg). 
     In the molded body, the aluminum nitride is present in the form of a single phase, homogeneous, isotropic microstructure having a maximum grain size of 5 μm. The residual oxygen and the residual carbon are present in the form of a solid solution in the aluminum nitride lattice and caromografically not detectable as separate phase(s) up to a magnification of 2400 times. The molded bodies have a bending strength measured according to the 4-point method, at room temperature and up to about 1400° C., of at least 500 N/mm 2 , a predominantly transcrystalline rupture modulus and a thermal conductivity at 300 K of at least 150 W/mK. The molded body is produced from a porous, deoxidized green body having a maximum density of 70% TD and the same chemical composition as the final product, by isostatic hot pressing in a vacuum sealed casing at a temperature of from 1700° to 2100° C. and a pressure of from 100 to 400 MPa in a high-pressure zone using an inert gas as a pressure transmitting agent.

Polycrystalline bodies of aluminum nitride are known. They arecharacterized by a combination of valuable properties such as highstrength, resistance to oxidation, resistance to thermal shock, highthermal conductivity, low electrical conductivity and resistance tocorrosion by liquid metals. Due to this combination of properties, theycan be used in many different fields. They are particularly useful asconstruction materials in high-temperature machinery and as substratematerials in high-efficiency electronic devices.

BACKGROUND OF THE INVENTION

The known polycrystalline aluminum nitride molded bodies however, havein part less desirable properties than pure aluminum nitride. Theproperties depend to a great extent on the amount of impurities that arepresent in the molded body and particularly, the amount of oxygen,carbon and metals present. For example, the theoretical value of thethermal conductivity of pure, monocrystalline aluminum nitride is 320W/mK, which drops to about 50 W/mK as the oxygen content increases.Still lower values are obtained on further increase of the oxygencontent or in two-phase aluminum nitride ceramics (see G.A. Slack in J.Phys. Chem. Solids (1973), Vol. 34, pp. 321-335; ref. in C.A. Vol. 78(1973), No. 129,310 r).

The strength at high temperatures also depends on the impurities presentin the molded body. The bending strength drops sharply at temperatureabove 1000° C. in comparison to the value measured at room temperaturewhich is ascribed to the presence of oxygen-containing phases at thegrain boundaries in the aluminum nitride sintered body.

Pure aluminum nitride does not sinter easily because of itspredominantly covalent bonding. In order to obtain high density bodies,it was deemed necessary either to start from aluminum nitride powdersrich in oxygen or to add sintering aids preferably metal oxides whichaid compression during hot pressing. It is possible to obtain fromaluminum nitride powder having an oxygen content of 1.0% by weight,(2.1% by weight based on Al₂ O₃) by axial hot pressing, a molded body ofaluminum nitride with 98% theoretical density (herein-after abbreviatedas % TD), which at room temperature has a bending strength of 265 N/mm²and which at 1400° C. dropped to 125 N/mm² (see DE-A-14 71 035corresponding to U.S. Pat. No. 3,108,887).

By hot pressing commercially available aluminum nitride powders at 2000°C., sintering densities of 97 to 99% TD were obtained. The purest of thepolycrystalline aluminum nitride molded bodies thus produced contained0.9% by weight oxygen, had a density of 97% TD and a thermalconductivity of 66 W/mK (see G.A. Slack et al. in Amer. Ceram. Soc.Bull. (1972) Vol. 71, pp. 852 to 856; ref. in C.A. Vol. 78 (1973) No.19686 K and DE-A-20 35 767).

Polycrystalline aluminum nitride molded bodies produced by hot pressingcommercially available aluminum nitride powder without sintering aids ata hot pressing temperature of 1700° C., had 0% porosity, contained 0.8%by weight oxygen and had a bending strength of 375 N/mm², measuredaccording to the 3-point method at room temperature, which at 1300° C.dropped to about 225 N/mm² (see P. Boch et al. in Ceram, Int. 1982, Vol.8 (1), pp. 34-40; ref. in C.A. Vol. 97(1982) No. 59846 m).

It has been reported from Japan, in relation to metallic impurities,that it is possible to hot press high-purity aluminum nitride powderswithout sintering aids at temperatures of 2000° C. to form densetransparent bodies. Physical data were given only for monophase aluminumnitride bodies produced with an admixture of 0.5 percent by weightcalcium oxide as a sintering aid and containing from 0.5 to 0.7% byweight oxygen. A hot pressed aluminum nitride body had a density of99.6% TD, a thermal conductivity of 91 W/mK and a bending strength of510 N/mm.sup. 2 measured at room temperature according to the 3-pointmethod. A pressureless sintered aluminum nitride body had a density of99.1% TD and a thermal conductivity of 95 W/mK (see N. Kuramoto et al.in J. Mater. Sci. Lett. 1984, Vol. 3 (6), pp. 471-474; ref in C.A. Vol.101 (1984), No. 42402s).

Aluminum nitride molded bodies produced by conventional hot pressingmethods, with biaxial application of pressure, have an anisotropicmicrostructure so that their properties depend on direction.

Since only bodies of simple shape can be produced by a hot pressingprocess, pressureless sintering processes have been developed for theproduction of polycrystalline aluminum nitride molded bodies.Pressureless sintering processes for aluminum nitride require use ofsintering aids to obtain high sintered densities. Numerous compoundshave been tested for promoting sintering of aluminum nitride. Especiallyeffective are oxides of elements from the 2nd and 3rd group of thePeriodic System including the lanthanides (see K. Komeya et al. in YogyoKyokaishi; 1981, Vol. 89 (6), pp. 330-336; ref. in C.A. Vol. 95 (1981),No. 155257 z).

Due to the sensitivity of aluminum nitride to impurities, in particularoxygen impurities, it is necessary to use the least possible amounts ofoxygen containing sintering aids or to reduce, by additional processingsteps, the oxygen present in the aluminum nitride powder and/or theoxygen introduced by the sintering aids.

According to the process disclosed in U.S. Pat. No. 4,435,513, a mixtureof commercially available aluminum nitride powders having an oxygencontent of not more than 5% by weight, together with up to 5.66% byweight of alkaline earth oxide sintering aids with up to 6.54% by weightcarbon in the form, for example, carbon black or a carbonizable organicmaterial such as sugar or phenolic resin were pressureless sintered attemperatures of up to 2000° C. The carbon in the mixture prevents theformation of aluminum oxide nitride phases and the amount of oxygenpresent in the starting aluminum nitride powder is reduced. As can beseen from the examples, the aluminum nitride molded bodies produced hada density of 98.5% TD and a thermal conductivity of 63 W/mK. By asubsequent hot isostatic pressing treatment, the density could beincreased to 99% TD and the thermal conductivity to 71 W/mk.

According to the process disclosed in EP-A-147 101, aluminum nitridepowders containing 0.001 to 7% by weight oxygen mixed with 0.01 to 15%by weight oxides of rare earth metals were hot pressed or sinteredwithout pressure. It is believed that the oxygen present in the aluminumnitride starting powder reacts with the oxides of the rare earth metals(preferably Y₂ O₃) forming compounds (phases) having a garnet orPerowskite structure so that oxygen does not diffuse into the aluminumnitride lattice with formation of mixed crystals or aluminum oxy-nitridephases (A1N polytypes). The garnet or Perovskite phases are formedduring sintering at relatively low temperatures (1000° to 1300° C.),they melt at high temperatures (1600° to 1950° C.) and induce aliquid-phase sintering that results in dense bodies. As can be seen fromthe examples, the best results with regard to thermal conductivity wereobtained with aluminum nitride bodies prepared from aluminum nitridepowders having oxygen contents of 0.3 to 1.0 percent by weight withadmixture of 0.1 to 3.0% by weight Y₂ O₃ which were pressurelesssintered at 1000° C. For an aluminum nitride (AlN) body with therelatively high oxygen content of approximately 0.9% by weight (0.6% byweight from the AlN powder +about 0.3 by weight oxygen from the 1.5% byweight Y₂ O₃), the highest heat conductivity given in all the exampleswas 135 W/mK. By X-ray diffraction analysis there were detected in thesebodies, together with the main aluminum nitride phase, small amounts ofan Al-Y garnet phase and an aluminum oxynitride phase, which are presentas oxidic impurities at the aluminum nitride grain boundaries.

According to the process disclosed in EP-A-13 32 75 (corresponding toU.S. Pat. No. 4,478,785 and U.S. Pat. No. 4,533,645), it was disclosedthat sintering aids were not necessary and only a carbon-containingmaterial was used. Commercially available aluminum nitride powders ofhigh purity, with regard to metallic impurities, and containingapproximately 1.5 to 3.0% by weight oxygen were partially deoxidized byadding carbon so that the aluminum nitride powders or the green bodyprepared therefrom still contained, after the deoxidation treatment byheating, a residual oxygen content of from about 0.35 to about 1.1%weight. The high residual oxygen content is necessary for pressurelesssintering, at temperatures in the range of 1900° to 2200° C., tosintered densities of more than 85% TD in resulting sintered bodies.Accordingly, the finished aluminum nitride sintered bodies have aresidual oxygen content in the range of about 0.35 to about 1.1% byweight and a residual carbon content in detectable amounts as low asabout 0.2% by weight. The sintered bodies of aluminum nitride preparedby the process are stated to be free of secondary phases which isunderstood to mean that they contain less than about 1% by volumesecondary phases (that is, phases other than AlN). As can be seen fromthe examples, however, the lack of sintering aids produces sinteredbodies with final densities in the range of 91.6 to 97.2% TD andrelatively high residual oxygen contents. In addition, despite the useof an aluminum nitride starting powder of high purity in relation tometallic impurities, the thermal conductivity values at room temperaturewere at a maximum of only 82 W/mK.

According to the process disclosed in EP-A-15 25 45, the improvement inthermal conductivity is obtained, by using for deoxidation of thealuminum nitride, an admixture containing yttrium such as yttrium metal,yttrium hydride and/or yttrium nitride instead of carbon. The yttriumreacts with the oxygen present in the aluminum nitride forming liquidphases containing yttrium and oxygen, which, at the same time act assintering aids in the pressureless sintering step. After cooling, thesephases remain in the aluminum nitride sintered body as secondary phasesat the grain boundaries of the aluminum nitride. The compositionaccording to point F in the phase diagram, has the smallest amount ofthese secondary phases, and corresponds to 1.6 equivalent percent Y and3.2 equivalent percent oxygen, which corresponds to 6.2% by weight of aYAlO₃ secondary phase, or, differently expressed, to 1.81% by weightoxygen and 3.36% by weight Y in the AlN sintered body. As it can be seenfrom the examples, the highest value for thermal conductivity was 174W/mK for an AlN sintered body containing Y₂ O₃ and Y.sub. 4 Al₂ O₉ assecondary phases.

As can be seen from the extensive prior art, it has not hitherto beenpossible to produce polycrystalline aluminum nitride bodies having ahigh density that do not contain substantial amounts of impurities suchas oxygen, carbon and/or metals, which unfavorably affect the thermalconductivity by changing the lattice parameters of the aluminum nitridecrystals and/or unfavorably affect resistance to high temperatures byinclusion of impurity containing phases at the grain boundaries of thealuminum nitride crystals.

BRIEF SUMMARY OF THE INVENTION

The problem is to provide molded bodies of polycrystalline aluminumnitride which are dense, substantially poreless and of high purity whichhave improved thermal and mechanical properties and therefore have abroad range of uses as construction materials in high-temperaturemachine construction and as substrate materials in high-efficiencyelectronics. In addition, a process whereby those molded bodies can beeconomically and reproducibly manufactured with the desired propertieswithout use of sintering aids is required.

According to the invention, the problem is solved by providing a densesubstantially non-porous molded body of polycrystalline aluminum nitridehaving a density of at least 99.8% TD calculated on the theoreticallypossible density of pure aluminum nitride and consisting of

at least 99% by weight aluminum nitride,

up to 0.35% by weight residual oxygen,

up to 0.35% by weight residual carbon and

up to 0.30% by weight total of metallic impurities (Fe, Si, Ca, Mg)

wherein the aluminum nitride is present essentially in the form of asingle, homogeneous, isotropic microstructure with a grain size notlarger than 5 μm; the residual oxygen and residual carbon are present inthe form of a solid solution in the AlN lattice and at an magnificationof up to 2,400 times, they are not ceramically detectable as separatephase(s), having the following properties: a bending strength (measuredaccording to the 4-point method) from room temperature to about 1400° C.of at least 500 N/mm², a predominantly transcrystalline rupture modulusand thermal conductivity at 300 K of at least 160 W/mK.

The molded bodies according to the invention are prepared from porousdeoxidized green bodies having a maximum density of 70% TD andconsisting of

at least 99% by weight aluminum nitride,

up to 0.35% by weight residual oxygen,

up to 0.35% by weight residual carbon and

up to 0.30% by weight total of metallic impurities (Fe, Si, Ca, Mg)

by isostatic hot pressing in a vacuum sealed casing at a temperature offrom 1700° to 2100° C. and a pressure of from 100 to 400 MPa in ahigh-pressure zone using an inert gas as a pressure-transmitting agent.

Since nothing can escape during the isostatic hot-pressing operation dueto the gas tight casing present, the molded bodies according to theinvention have at least 99.8% TD, preferably 100% TD, and the samechemical composition as the porous deoxidized green bodies with amaximum 70% TD.

DETAILED DESCRIPTION OF THE INVENTION

The molded bodies according to the invention, made of polycrystallinealuminum nitride, have a single phase microstructure in which theindividual AlN grains having a maximum grain sizes of 5 μm aredistributed uniformly, that is, homogeneously and independently ofdirection. The residual oxygen and residual carbon are essentially inthe form of a solid solution in the aluminum nitride lattice and cannotbe detected as a separate phase or phases by x-ray diffraction or byceramographic techniques at magnifications of up to 2400 fold. Themetallic impurities, however, can be detectable in the form ofparticlate segregations in sizes of ≦0.5 μm.

For the production of the deoxidized green bodies, it is preferable touse as the AlN starting material, a powder having a maximum particulesize of 5 μm, preferably 2 μm, and an average particle size of <1 μm,preferably <0.5 μm, with a specific surface of from 4 to 10 m² /g(measured according to BET) and a purity of at least 99.8%, preferably99.9%, calculated on the metallic impurities. Metallic impurities are tobe understood to mean all metallic elements (essentially Fe, Si, Ca andMg), with the exception of the aluminum present in bonded form, whichcan be present in AlN powders.

The adherent carbon present in commercially available AlN powder can betolerated to a maximum of 0.2% by weight. The residual oxygen which as aresult of the known tendency of the finely divided AlN powder tohydrolyze (according to AlN+3H₂ O →NH₃ +Al(OH)₃) is present as the mainimpurity mostly in the form of the hydrolysis product Al(OH)₃, can betolerated up to a maximum of 4.0% by weight.

The aluminum nitride starting powders in admixture with small amounts offree carbon or a material which forms carbon on heating is compacted toform pre-molded green bodies and then subjected to a heat treatment,which is a purifying deoxidizing annealing at 1600° to 1800° C. to anitrogen atmosphere, to form deoxidized green bodies with a maximumdensity not exceeding 70% TD.

The carbon-containing admixture for the preparation of the deoxidizedgreen bodies can be formed in any manner that ensures a uniformdistribution of the carbon in the AlN-C mixture, for instance, byadmixture of the AlN with particulate carbon black or colloidal graphitewith a specific surface in the range of from 10 to 400 m² /g. To obtaingood pressing properties of the powder mixtures containing carbon blackor colloidal graphite, it is preferable to use small amounts of atemporary binder such as camphor or stearic acid. The temporary bindersare preferably used in amounts of up to a maximum of about 3% by weightcalculated on the resulting mixture. The admixture containing carbonpreferably contains an organic material that can be carbonized attemperatures of up to about 1000° C. forming carbon. Examples ofpreferred organic materials are condensation products of phenolformaldehyde of the Novolak and Resole type which are carbonized in therange of from 100° to 900° C. forming amorphous carbon in yields of 35to 50%.

For determining the amount of carbon to be admixed with the startingaluminum nitride powder, the free carbon present in the aluminum nitridestarting powder has to be taken into consideration. The total amount ofthe free carbon present in the compacted powder mixture, aftercarbonizing the organic material if an organic material is used, iscritical for carrying out the process and for obtaining the advantageousproperties of the sintered bodies of the invention. It has been foundthat more carbon must be used than is stoichiometrically required fordeoxidation of the oxygen impurities present in the aluminum nitridepowder. As a calculation basis for determining the stoichiometricallyrequired amount of carbon, the following chemical equation for thecarbothermal reduction of aluminum hydroxide in a nitrogen atmospherecan be used.

    2 Al(OH).sub.3 +3 C+N.sub.2 →2 AlN+3 H.sub.2 O+3 CO.

However, the value thus calculated is an approximate value since theoxygen in the aluminum nitride powder as a rule is not presentcompletely as Al(OH)₃ but partly as physically or chemically absorbedwater, as oxygen dissolved in the aluminum nitride lattice and asaluminum oxide (Al₂ O₃).

The amount of the carbon admixed with the AlN powder is preferably usedin an amount sufficient to lower the oxygen content in the preoxidizedgreen body to less than 0.35% by weight residual oxygen but which at thesame time does not increase the carbon content of the deoxidized greenbody to more than 0.35% by weight residual carbon. The addition of ainsufficient amount of carbon results in deoxidized green bodies havingmore than 0.35% by weight oxygen; the addition of too large amounts ofcarbon results in deoxidized green bodies having more than 0.35% byweight carbon; in either case, the properties of the dense aluminumnitride molded bodies produced from the deoxidized green bodies areadversely effected. The optimal amount of the carbon admixed withaluminum nitride starting powder of specific grain fineness and a givencontent of oxygen and carbon can be easily ascertained by performingdeoxidation tests.

The process for producing deoxidized green bodies is as follows:

The AlN powder is first homogeneously mixed with the carbon-containingmaterial which is preferably obtained by dissolving a carbon containingorganic material in a solvent and dispersing the AlN powder in thesolution. When free carbon per se is used, the AlN powder together withthe elementary carbon can be dispersed in a solution of a temporarybinder. Useful organic solvents, include acetone and lower alcoholshaving 1 to 6 C atoms. The dispersion can be carried out by stirring adilute suspension in a plastic container using a stirrer or by kneadinga viscous suspension in a kneading apparatus. The solvent can beremoved, for instance, in the case of a dilute suspension, by spraydrying or, in the case of a viscous suspension, by evaporation duringthe kneading operation. Generally, the dried material is milled in a jetmill, pin beater mill or ball mill to disintegrate agglomerates toensure homogeneous distribution of the carbon-containing material in theadmixture.

The starting powder mixtures are compacted by molding to form pre-moldedgreen bodies. The molding can be effected by means of conventionallyknown steps such as die pressing, isostatic pressing or slip casting. Incase of die pressing or isostatic pressing, a pressure between 10 and200 MPa, preferably 50 to 100 MPa is generally applied.

The pre-molded green bodies are then subjected, according to theinvention, to an deoxidation annealing under a nitrogen atmosphere at1600° to 1800° C. The indicated temperature range is critical forobtaining the desired properties of the final product. It has been shownthat under equivalent conditions but at lower temperatures, insufficientdeoxidation was obtained and the residual oxygen content was above 0.35%by weight whereas at higher temperatures especially 1900° C., as aconsequence of partial sintering, a noticeable grain enlargementoccurred which is associated with a deterioration of the strengthproperties of the final product.

The deoxidation annealing of the pre-molded green bodies can be carriedout in any desired high-temperature apparatus such as a graphite tuberesistance furnace (Tammann furnace) or in an inductively heated furnacewith graphite suszepfor. For continuous operation, there can beadvantageously used a horizontal pusher or band-type furnace in whichthe pre-molded green bodies are transported through the hot zone of thefurnace in a manner such that they can each be held at the desiredtemperature for a predetermined period of time. The time intervals forheating up and dwelling at the final temperature are here dependent onthe size of the pre-molded green bodies to be deoxidized. The pre-moldedgreen bodies are conveniently accommodated in graphite containers andsurrounded by coarse-grained aluminum nitride powder to preventcarburization from the graphite container. But the pre-molded greenbodies are preferably loaded in containers of aluminum nitride withoutusing the surrounding powder bed of aluminum nitride. Nitrogen,optionally mixed with carbon monoxide, is used as the gaseousatmosphere. The deoxidation is advantageously carried out in a flowingnitrogen atmosphere that is, under a nitrogen pressure of about 0.1 MPa;but it can also be carried out under reduced N₂ pressure, a pressure ofabout 5000 Pa having proved especially satisfactory. The deoxidizedgreen bodies obtained after the deoxidation annealing have, as a rule, adensity of 55 to 65% Td, but in all cases ≦70% TD, that is, they areporous with open porosity whereby it is to be understood that they havecanal pores which are intercommunicating and open to the surface of thearticle.

According to the present invention, these deoxidized green bodiesconsist of at least 99% by weight AlN, with residual oxygen and residualcarbon contents preferably of less than 0.35% by weight each and withunavoidable metallic impurities preferably amounting to less than 0.3%by weight. The metallic impurities present in the aluminum nitridepowder are exclusively from the preparation and further processing ofthe powder since no metal-containing sintering aids are intentionallyadded to the carbon containing aluminum nitride powder mixtures.

The deoxidized green bodies are used for the preparation of thesubstantially non-porous dense molded bodies of polycrystalline aluminumnitride. According to the invention, the deoxidized green bodies areisostatically hot pressed in an gastight casing at a temperature of from1700° to 2100° C. and a pressure of from 100 to 400 MPa in ahigh-pressure autoclave using an inert gas as a pressure transfermedium.

For carying out the process of the invention, for preparation of themolded bodies, the deoxidized green bodies must be provided with agastight casing before being introduced into the high-pressure zone soas to prevent the gas, used as the pressure-transmitting agent, topenetrate through the open pores into the body thereby hinderingcompression.

The casing must be formed from materials that can be sealed gastight andwhich at the applied pressing temperatures in the range of from 1700° to2100° C. neither melt nor react with the deoxidized green bodies thatis, they must remain inert in respect to the deoxidized green bodies.The casing must be sufficiently plastic at the pressing temperature usedto adapt to the shape of the body without cracking to ensure that thegaseous pressure is uniformly transmitted via the casing to the body.

Examples of suitable casing materials that meet the requirements includehigh-melting glasses such as pure quartz glass, high-melting ceramics orhigh-melting metals and metal alloys like molybdenum, tantalum ortungsten. These materials can be used in the form of pre-fabricatedcasings or capsules in which the deoxidized green bodies are introduced.The casings together with the contents are then evacuated and sealedgastight. The casings can also be formed on the deoxidized green bodiesby direct coating, for instance, by electroless deposition of metals orby applying a glass-like or ceramic-like composition, which is thenmelted or sintered in vacuum to form the gastight casing. In addition,it is preferred to apply between the casing and the deoxidized greenbody to be compressed, an intermediate layer. The intermediate layer cancomprise inert powders, fibers or felts, for example, graphite feltsand/or boron nitride powder. In addition, bodies provided with a casingof high-melting glass can be embedded in a powder bed of fineparticulate material that serves to reinforce the glass casing from theoutside. The expression "vacuumtight sealed casing" is intended to referto a casing which is impervious to the pressure gas acting from theoutside and which contains no residual gases that disturb thecompression operation.

The deoxidized green bodies provided with gas-tight sealed casings areconveniently housed in graphite containers, then introduced in to thehigh-pressure zone and heated to the required compression temperature ofat least about 1700° C. It is convenient here to separately regulatepressure and temperature that is, to raise the gaseous pressure onlywhen the casing material starts to plastically deform at the elevatedtemperature. Argon or nitrogen are preferably used as inert gases forthe transmission of pressure. The gaseous pressure applied is preferablewithin the range of 150 to 250 MPa reached by slow intensification atthe final temperature used which is preferably within the range of 1800°to 2000° C. The optimal temperature within the range of 1700° to 2100°C. depends on the fineness and purity of the aluminum nitride startingpowder used and on the chemical composition of the deoxidized greenbodies. The maximum temperature of about 2100° C. should not be exceededsince there is danger that the non-porous molded bodies formed wouldacquire a "secondary recrystallized microstructure" that reduces thestrength and is no longer homogeneous since some grains become largerthan the rest.

After lowering the pressure and temperature, the cooled bodies areremoved from the high-pressure autoclave and the casing is removed fromthe dense body. The casing can be removed by milling or fusing the metalcasing by sandblasting glass or ceramic casings or by chemicalcorrosion.

The molded bodies thus produced are substantially non-porous with adensity of at least 99.8% TD and are substantially texture-free asresult of the uniform multidirectional action of pressure that is, theyhave an isotropic microstructure and their properties are not dependenton direction but are substantially the same in all directions. Thebending strength used for characterizing the high-temperature strengthis not unfavorably affected by secondary phases at the grain boundariesby sintering aids. Bending strength values have been obtained whichare >500 N/mm², preferably >600 N/mm² that are not substantially reducedup to 1400° C.

The absence of texture, the extremely fine-grained microstructure whichis practically single phase with maximum grain size of 5 μm, preferably2 μm, and the occurrence of a transcrystalline fracture mode areresponsible for the excellent mechanical strength properties.

The fracture mode of the molded bodies is transcrystalline up to atemperature of about 1370° C. It is thus ensured that the grainboundaries do not provide a defect area for reducing strength that is,under stress at elevated temperature sliding of the grains at the grainboundaries is suppressed so that the molded bodies have high strengthunder long-term stress and a high creeping resistance. As result oftheir purity and of the practically 100% theoretical density, the moldedbodies possess an extraordinary electrical insulating capacitycorresponding to a specific electrical resistance of 10¹⁴ ohm×cmtogether with excellent thermal conductivity of at least 150 W/mK,preferably >200 W/mK.

Accordingly, molded bodies of polycrystalline AlN of the invention havea better range of properties than those produced according to knownprocesses of pressureless sintering or hot pressing with or without useof sintering aids. The process for production of molded bodies by hotisostatic pressing is not as limited with regard to molding possibilityas conventional hot pressing. A high pressure autoclave can contain alarge furnace zone where numerous encased samples of any desired shapecan be simultaneously hot isostatically pressed. From the hotisostatically pressed AlN bodies, thin substrate wafers can be producedat reasonable cost by conventional machining methods such as with aninner-hole saw. In particular, the combination of the followingproperties: high thermal conductivity, high electrical insulatingcapacity, low expansion coefficient and high thermal-shock resistance,recommends the material according to the invention made of dense, pureAlN for use as a substrate in high-efficiency electronics as, forexample, as small mounting plates for semi-conductor elements orelectronic circuits. The low specific weight, the good resistance tohigh temperatures and good thermal conductivity also make possible,however, their use as structural materials in high-temperature machinerysuch as engine construction.

The process for producing the molded bodies according to the inventionis explained in detail with reference to the examples that follow. Therelative densities in % TD given in the specification and in theexamples for the molded bodies have been calculated on the basis of thetheoretical density of 3.26 g/cm³ of the aluminum nitride.

EXAMPLE 1

A technically pure AlN powder having a specific surface of 8.9 m² /g wasused as the starting material. The chemical analysis of this powder,which had a maximum particle size of 1 μm, is disclosed in Table 1. Acommercially available, powdery phenol formaldehyde resin of the Novolaktype (for instance ALNOVOL® of the firm Hoechst AG) was used as thecarbon-containing material. To each 100 parts by weight of the AlNpowder, 1.75 parts by weight of Novolak powder in the form of a solutionin acetone were added and the viscous slurry was dried in air until thesolvent had evaporated. The crumbly powder obtained after kneading wasdeglomerated by dry grinding in a jet mill and then isostaticallycompressed under a pressure of 100 MPa to cylinders 30φ×50 mm (30 mmdiameter×50 mm height).

The cylindrical blanks were then annealed for two hours at 1800° C.under a flowing nitrogen atmosphere under a gaseous pressure of 0.1 MPain an AlN crucible that had been introduced in the hot zone of agraphite furnace of the Tammann type. The annealing was carried outaccording to the following temperature schedule:

20°-400° C. : 60 min.

400°-1800° C. : 120 min.

kept at 1800° C. : 120 min. At the end of the dwell period, the furnacewas switched off and the deoxidized green bodies were cooled in thefurnace to room temperature. The deoxidized green bodies had a greendensity of an average 66% TD, a residual oxygen content of 0.29% byweight and a residual carbon content of 0.23% by weight. As shown inTable 1 which sets forth the analyses of the AlN starting powder and thedeoxidized green body, practically no change occurs with regard tocarbon content and metallic impurities whereas the oxygen content isdrastically lowered from 1.80 to 0.29% by weight that is, above 80%based on the oxygen content of the starting powder.

                  TABLE 1                                                         ______________________________________                                        Analysis of the AlN sintering powder and                                      of the green body deoxidized at 1800° C.                                         AlN sintering powder                                                                         deox. green body                                     Elements  (% by weight)  (% by weight)                                        ______________________________________                                        N         32.9           33.8                                                 O         1.80           0.29                                                 C         0.21           0.23                                                 Fe        0.139          0.210                                                Si        0.029          0.031                                                Ca        0.006          0.005                                                Mg        0.003          0.004                                                ______________________________________                                    

The deoxidized green bodies were then introduced into prefabricatedquartz glass casings and the space between the inrer side of the casingand the green body filled with finely divided boron nitride powder. Thecasing together with the contents were then evacuated, heated to 1000°C. in vacuum and sealed gastight by melting in an oxyhydrogen burner.The encased samples were then hot isostatically compressed at 1800° C.in a high-pressure autoclave under an argon gaseous pressure of 200 MPa.The hot isostatic pressing was scheduled according to the followingtemperature/pressure program:

20°-800° C./0.1 MPa : 60 min.

800°-1400° C./0.1 MPa : 60 min.

1400°-1600° C./0.1-125 MPa : 120 min.

1600°-1800° C./125-200 MPa : 120 min.

kept at 1800° C./200 MPa : 60 min.

1800°-1350° C./200 MPa : 60 min.

1350°-1250° C./200-5 MPa : 30 min.

After decompressing and cooling the molded bodies to room temperature,the samples were removed from the hot isostatic pressing equipment andthe glass casings removed by crushing and sandblasting. The AlN moldedbodies thus produced had a density totally of 3.26 g/cm³, whichcorresponds to 100% of the theoretical density. After carring out thedensity measurements and surface grinding, cylindrical test bodiesmeasuring 20φ×28 mm and 20φ×1 mm and prismatic small test bars 2×4×34 mmfor determining the thermal conductivity, the specific electricresistance and the bending strength were produced from the moldedbodies. The thermal conductivity was determined according to thecomparative rod method up to 927° C. using Armco iron as the referencematerial. The thermal conductivity of the AlN samples in function of thetesting temperature is given in Table 2.

                  TABLE 2                                                         ______________________________________                                        Testing temperature                                                                             Heat conductivity                                           (°C.) (K)      (W/mK)                                                  ______________________________________                                         27          300      161                                                     177          450      101                                                     327          600      76                                                      477          750      61                                                      627          900      50                                                      777          1050     43                                                      927          1200     38                                                      ______________________________________                                    

A value of 10¹⁴ ohm×cm was obtained for the specific electric resistancewhich was measured at room temperature (25° C.) with direct currentaccording to the three-point measuring method. The bending strength ofthe test body was measured according to the four-point method usingsupport distances of 15 mm (upper) and 30 mm (lower). The test bars,which break in a transgranular manner, had the following average bendingstrength value at room temperature (average value from 5 measurements):621 N/mm². A value of 635 N/mm² was obtained for the bending strength of1370° C. The microstructure of the samples was, according to results ofx-ray and microstructural analyses, single phase with a maximum grainsize of 2 μm.

EXAMPLES 2 AND 3 (FOR COMPARISON)

Example 1 was repeated with the variation that at one time, no carbonwas admixed with the AlN powder (Example 2) and one time, an excess ofcarbon was admixed with the AlN powder (Example 3). The green bodies inExample 2 were pressed using 2% by weight camphor in the form of asolution in acetone as a temporary binder which was removed withoutresidue during the heating operation in the deoxidation annealing step.Table 3 gives a characterization of the deoxidized green bodies and theproperties of AlN bodies made therefrom by hot isostatic pressing inquartz glass casings, as indicated in Example 1.

                                      TABLE 3                                     __________________________________________________________________________    Analyses results of the deoxidized green bodies and properties of hot         isostatically pressed molded bodies made therefrom                                                  hot isostatically pressed molded body                   deoxidized green body                σ.sub.B.sup.+++                    Example                                                                            density                                                                            residual O                                                                          residual C                                                                          density                                                                            Λ.sup.+                                                                     ρ.sup.++                                                                       (N/mm.sup.2)                                                                           Rupture                         No.  (% TD)                                                                             (% wt)                                                                              (% wt)                                                                              (% TD)                                                                             (W/mK)                                                                             (Ω · cm)                                                            25° C.                                                                     1370° C.                                                                    mode                            __________________________________________________________________________    2    59   1.26  0.21  100   98  10.sup.11                                                                          656 459  i                               3    66   0.18  0.62  100  122  10.sup.12                                                                          550 581  t                               __________________________________________________________________________     .sup.+ Λ . . . thermal conductivity measured at room temperature       (25° C.)                                                               i,t . . . Intercrystalline or transcrystalline                                .sup.++ ρ . . . electrical resistivity measured at 25° C.          .sup.+++ σ.sub.B . . . bending strength measured at 25 and              1370° C.                                                          

As seen from the data in Table 3, by hot isostatic pressing of encasedporous AlN green bodies made from the AlN powder without addition ofcarbon for deoxidizing the oxygen impurities, AlN bodies according tothe invention are not obtained. Although the bodies are compressed to100% of the theoretical density of AlN and possess excellent strength atroom temperature; as a result of their high content of residual oxygen(see Table 3, Example 2), the values of heat conductivity, specificelectrical resistance and high-temperature strength are clearly inferiorto those of Example 1. The fractured surfaces of the samples show bothat room temperature and at 1370° C. an intercrystalline rupture mode,which, together with the drastic drop of strength at 1370° C., can beascribed to the presence of an oxygencontaining grain boundary phase.

It clearly appears from the results set forth in Table 3, for Example 3,that when a large excess of carbon is admixed with the AlN powder whichin this case provided a carbon content of 0.62% by weight in theannealed green body, the AlN molded bodies according to the inventionwere likewise not obtained. Even though these bodies have low contentsof residual oxygen and a transcrystalline rupture mode, the high levelsof high-temperature and room-temperature strength, thermal conductivityand electrical insulating capacity, are not obtained in the dense AlNbodies.

EXAMPLES 4-5

Example 1 was repeated with the essential changes that follow:

(1) a different AlN powder with regard to purity and particle size wasused;

(2) the carbon was in the form of elemental carbon;

(3) the deoxidizing annealing was carried out under a nitrogenatmosphere with a pressure of 5000 Pa and a final temperature of 1400°C. (Example 4) and 1700° C. (Example 5),

(4) the deoxidized green bodies were encased in vacuum sealed molybdenumcapsules and finally

(5) 2000° C. was selected as the final temperature for the hot isostaticpressing.

The chemical analysis of the AlN sintering powder, which had a specificsurface of 5.6 m² /g and a maximum particle size of 3 μm, is shown inTable 4. Carbon black with a specific surface of 150 m² /g in an amountcorresponding to 0.63 g calculated on 100 g AlN powder was the elementalcarbon used. To improve the pressability, in analogy to Example 2, theAlN-C mixture was further processed with a camphor solution to form apowder for pressing. Table 4 discloses the analyses of the AlN sinteringpowder and the deoxidized green bodies annealed at 1400° C. and 1700° C.The data in Table 4 shows that deoxidation annealing at 1400° C.(Example 4), contrary to Example 5, does not provide sufficientdeoxidation. The deoxidation reaction stopped under the selectedconditions with respective contents of residual oxygen and residualcarbon above the required limit of 0.35% by weight.

                  TABLE 4                                                         ______________________________________                                        Analysis of the AlN sintering powder and of the green                         bodies deoxidized at 1400° C. and at 1700° C.                                     Green bodies deox. at                                       AlN sintering powder                                                                              1400° C.                                                                        1700° C.                                  Elements                                                                             (% weight)       (% weight)                                            ______________________________________                                        N      33.1              n.d.*   34.0                                         O      1.67             0.41     0.15                                         C      0.11             0.39     0.12                                         Fe     0.002            n.d.     0.075                                        Si     0.010            n.d.     0.020                                        Ca     0.002            n.d.     0.003                                        Mg     0.001            n.d.     0.002                                        ______________________________________                                         n.d. = not determined                                                    

The green bodies annealed at 1400° and 1700° C. were encased invacuum-sealed molybedenum capsules by welding and after the hotisostatic pressing cycle which was carried out similarly as indicated inExample 1 but at a final temperature of 2000° C., opened with the helpof an electron welding apparatus.

The hot isostatically pressed AlN bodies, after removing the bodies fromthe casing, were analysed with regard to their density, theirmicrostructure and values for thermal conductivity and specific electricresistance, as already described in Example 1. The results of thesemeasurements are set forth in Table 5.

                  TABLE 5                                                         ______________________________________                                                        Heat con-                                                                              spec. electr.                                                        ductivity                                                                              resistance                                                                             Grain size of                               Example                                                                              Density  at 25° C.                                                                       at 25° C.                                                                       microstructure                              No.    (% TD)   (W/mK)   (ohm · cm)                                                                    μm                                       ______________________________________                                        4      100       90      10.sup.13                                                                              <3                                          5      100      206      10.sup.14                                                                              <4                                          ______________________________________                                    

Thus, according to Example 5, after practically complete deoxidationthat is, by hot isostatic pressing of green bodies deoxidized down tocontents of residual oxygen and residual carbon of ≦0.15% by weight, thethermal conductivity of non-porous polycrystalline AlN can be increasedto more than 200 W/mK. The higher thermal conductivity obtained in thecomparison of Example 1 can be attributed not only to the extremely lowcontents of non-metallic impurities (oxygen and carbon), but also to thedegree of purity of the deoxidized green bodies of about 99.9% based onthe metallic impurities.

The influence of the content of residual oxygen and residual carbon ofmore than 0.35% on the heat conductivity and electric resistance isclearly shown from a comparison of Examples 4 and 5. Despite the use ofa very pure AlN sintering powder having a purity of more than 99.9%based on the metal impurities, an AlN body of the invention was nolonger obtained in Example 4.

It is observed that pure AlN sintering powder can be compressed withoutusing the hot isostatic pressing technique and without other additiveswhich promote sintering according to the conventional (axial) hotpressing process with graphite dies which form non-porous AlN moldedbodies having a density of 3.26 g/cm³, but with thermal conductivitiesbelow 100 W/mK.

We claim:
 1. A molded body of polycrystalline aluminum nitride having adensity of at least 99.8% TD calculated on the theoretical density ofpure aluminum nitride comprising:at least 99% by weight aluminumnitride, up to 0.35% by weight residual oxygen, up to 0.35% by weightresidual carbon and up to 0.30% by weight total of metallic impurities(Fe, Si, Ca, Mg),wherein the aluminum nitride is present in the form ofan essentially single-phase, homogeneous, isotropic microstructure witha grain size of 5 μm maximum, the residual oxygen and the residualcarbon being present in the form of a solid solution in the AlN latticeand not detectable as separate phase(s) up to a 2400-times enlargement,having the following properties: bending strength (measured according tothe 4-point method) at room temperature and up to about 1400° C. of atleast 500 N/mm², predominantly transcrystalline rupture module and aheat conductivity at 300 K of at least 150 W/mK.
 2. A molded bodyaccording to claim 1 produced from a porous, deoxidized green bodyhaving a maximum density of 70% TD consisting ofat least 99% by weightaluminum nitride, up to 0.35% by weight residual oxygen, up to 0.35% byweight residual carbon and up to 0.30% by weight total of metallicimpurities (Fe, Si, Ca, Mg),obtained by isostatic hot pressing saidgreen body in a vacuum sealed casing at a temperature of from 1700° to2100° C. and a pressure of from 100 to 400 Mpa in a high-pressureautoclave with an inert gas as a pressure transmitting agent.